Oxidation of Dimethylselenide by .delta.-MnO2: Oxidation Product and Factors Affecting Oxidation Rate

Abstract

Volatile dimethylselenide (DMSe) was transformed to a nonvolatile Se compound in a δ-Mn0 2 suspension. The nonvolatile product was a single compound identified as dimethylselenoxide based on its mass spectra pattern. After 24 h, 100% of the DMSe added to a δ-MnO 2 suspension was converted to nonpurgable Se as opposed to 20%, 18%, and 4% conversion for chromate, permanganate, and the filtrate from the suspension, respectively. Manganese was found in solution after reaction. These results imply that the reaction between manganese oxide and DMSe was a heterogeneous redox reaction involving solid phase δ-MnD 2 and solution phase DMSe. Oxidation of DMSe to dimethylselenoxide [OSe(CH 3 ) 2 ] by a δ-MnO 2 suspension appears to be first order with respect to δ-MnO 2 , to DMSe, and to hydrogen ion with an overall rate law of d[OSe(CH 3 ) 2 ]/dt = 95 M -2 min -1 [MnO 2 ] 1 [DMSe] 1 [H + ] 1 for the MnD 2 concentration range of 0.89 x 10 -3 - 2.46 x 10 -3 M, the DMSe concentration range of 3.9 x 10 -7 - 15.5 x 10 -7 M Se, and a hydrogen ion concentation range of 7.4 x 10 -6 - 9.5 x 10 -8 M. A general surface site adsorption model is consistent with this rate equation if the uncharged OMnOH is the surface adsorption site. DMSe acts as a Lewis base, and the manganese oxide surface acts as a Lewis acid. DMSe adsorption to OMnOH can be viewed as a Lewis acid/base complex between the largely p orbitals of the DMSe lone pair and the unoccupied e g orbitals on manganese oxide. For such a complex, frontier molecular orbital theory predicts electron transfer to occur via an inner-sphere complex between the DMSe and the manganese oxide.